1. Field of the Invention
This invention relates to plasma processing equipment and, more particularly, to downstream plasma reactor systems usable in semiconductor processing.
2. Description of the Related Art
The information described below is not admitted to be prior art by virtue of its inclusion in this Background section.
Plasma processing is commonly used in semiconductor fabrication. One use for plasma processing is in the removal of layers formed on a substrate, typically by etching some or all of a particular layer. Plasma processing is often performed in single chamber reactor systems in which plasma is generated exclusively in the chamber in which processing is carried out. Alternatively, downstream plasma reactor systems may be used that first convert gases into plasma in a plasma tube and then transport the plasma-generated reactive species downstream into the reaction chamber. These reactor systems can be used to avoid the radiation damage and resist hardening common in single chamber plasma reactor systems. And like single chamber plasma reactor systems, downstream plasma reactor systems can be used to create reactive species capable of etching layers of silicon dioxide, silicon nitride, aluminum, and various other materials commonly used in semiconductor fabrication.
A common application for downstream plasma reactor systems is resist stripping, i.e., the removal of patterned photoresist after completion of an etch step. Resist stripping usually is carried out in an ashing process in which the resist is oxidized to a gaseous form and removed from the reaction chamber. Those downstream plasma reactor systems that are specifically configured for resist stripping are often labeled downstream plasma strippers.
FIG. 1 presents a schematic view of an exemplary downstream plasma reactor system 100, the GaSonics L3500, which is commercially available from GaSonics International (San Jose, Calif.). Because it is primarily configured to remove resist, downstream plasma reactor system 100 may be properly labeled a downstream plasma stripper. Reactor system 100 includes a plasma tube 104. Plasma tube 104 is made up of an intake portion 106, a central portion 108, and a discharge portion 110. Gas source 102 is configured to be in gaseous communication with (i.e., may be operably connected such that gases can flow therebetween) intake portion 106. Plasma tube 104 is coupled to inlet conduit 112. Inlet conduit 112 is connected to reaction chamber 114. Reactor system 100 also includes plasma generating apparatus 111, which is positioned adjacent to plasma tube central portion 108 and includes a power supply and a microwave generator. Outlet conduit 116 is connected to reaction chamber 114 and is configured to be in selective gaseous communication with vacuum pump 118.
During operation of downstream plasma reactor system 100, vacuum pump 118 may be used to evacuate gases from reaction chamber 114 and all conduits in gaseous communication with reaction chamber 114, including inlet conduit 112 and plasma tube 104. Gases may be introduced into plasma tube 104 from gas source 102 via intake portion 106. The desired amounts and proportions of gases supplied by gas source 102 may be regulated using one or more mass flow controllers. These gases are typically selected such that the reactive species generated upon plasma formation are appropriate for the particular process being performed. As the gases enter central portion 108, microwaves created by plasma generating apparatus 111 convert at least a portion of the entering gases into plasma (i.e., creating a partially ionized plasma). The plasma generated in central portion 108 subsequently passes into discharge portion 110. From discharge portion 110, the plasma is conveyed into inlet conduit 112. The plasma is then transported through inlet conduit 112 into reaction chamber 114 to be used in processing.
FIG. 2 presents an expanded cross-sectional view of section A of reactor system 100. Section A includes parts of discharge portion 110 of plasma tube 104 and coupling portion 126 of inlet conduit 112. As shown in FIG. 2, discharge portion 110 may be subdivided into an initial section 120, an expanded section 122, and a tube extension 124. As shown in FIG. 2, tube extension 124 is a tube positioned partially within and extending beyond initial section 120 and expanded section 122. Initial section 120, expanded section 122, and tube extension 124 are composed of fused quartz, and tube extension 124 is fixably attached (i.e., coupled such that it is not capable of significant independent movement) to initial section 120 at glass weld 123. Discharge opening 125 is defined at the end of tube extension 124. Sealing o-ring groove 128 is defined within expanded section 122 and is configured to hold sealing o-ring 130. Sealing o-ring 130 is composed of an elastomeric material. Being composed an elastomeric material, sealing o-ring 130 is able to conform to the surfaces of plasma tube 104 and inlet conduit 112 to achieve good sealing action. Sealing o-ring 130 should be configured to make a seal between plasma tube 104 and inlet conduit 112 sufficient to maintain the level of vacuum desired within the plasma tube and inlet conduit. Coupling section 126 includes socket 131 and throat 133. Socket 131 is configured to fit snugly around expanded section 122 when sealing o-ring 130 is in place.
Sealing o-ring 130 should not only provide a good seal between plasma tube 104 and inlet conduit 112, but should also be able to maintain such a seal over numerous operation cycles carried out over a sizable time period. One factor that determines the life of a seal over repeated operational cycles is whether the seal posses sufficient resiliency. Sufficient resiliency in sealing o-ring 130 is important because when reactor system 100 is under vacuum during an operation cycle, coupling portion 126 exerts substantial lateral (i.e. radial) force on the sealing o-ring, compressing it. Then when then the cycle is completed, vacuum is released and the lateral force exerted by coupling section 126 subsides. A sufficiently resilient sealing o-ring 130 is able to be significantly compressed during a vacuum cycle and then return to its original shape after completion of the cycle. As such, a sufficiently resilient sealing o-ring may maintain high seal quality over numerous operation cycles.
With time, repeated compression and expansion can cause even the most resilient of o-rings to fail; however, it is desired that the mean time between failures of a sealing o-ring be extended as long as is reasonably possible. For example, replacing sealing o-ring 130 requires the purchase of a new o-ring and necessitates the expenditure of limited employee time. Over time, the reduction in the throughput of reactor system 100 during these replacement periods can result in a substantial loss of production value. It is thus beneficial to reduce the frequency with which replacement of sealing o-ring 130 is required (i.e., to extend the mean time between failure for the o-ring).
Unfortunately, the operating conditions of reactor system 100 can greatly reduce the amount of time between failures of sealing o-ring 130. One explanation for this outcome is the presence of numerous reactive species (e.g., ions and radicals) in the plasma exiting the plasma tube. Most of these reactive species will pass directly into the inlet conduit, but some end up in contact with sealing o-ring 130. While these plasma-generated reactive species do not substantially erode the fused quartz of which plasma tube 104 is constructed, other elements of the plasma system, such as sealing o-ring 130, arc often constructed of materials more susceptible to such erosion. Furthermore, resist stripping often incorporates hydrogen- and oxygen-containing plasmas that have a particularly pronounced ability to degrade many commonly used sealing materials. As such, the chemical resistance of sealing o-ring 130 to plasma-generated radicals can greatly influence the average time between failure of such an o-ring.
This problem may be partially resolved by the use of a tube extension 124 such as is shown in FIG. 2. One purpose of tube extension 124 is to provide a clean flow of plasma from discharge opening 10 into throat 133. In other words, tube extension 124 serves to increase the length that the plasma travels beyond sealing o-ring 130 so that less of the plasma will be able to double back and attack the o-ring. But since tube extension 124 is composed of fused quartz it is relatively inflexible (e.g., it cannot undergo substantial bending without breaking and/or cracking), and as such cannot adequately seal throat 133 of coupling section 126. Consequently, a large number of plasma-generated reactive species are still able to reach sealing 130. The reactive species can quickly erode non-chemically resistant sealing o-rings, making such o-rings almost unusable as sealing o-ring 130.
In an attempt to overcome such difficulties, numerous chemically resistant elastomers have been used as materials for sealing o-ring 130. One of these is Viton.RTM., a fluoroelastomer commercially available from DuPont Dow Elastomers, Wilmington, Del. Viton.RTM. has good resiliency, and is suitable for use in vacuum operations. But while Viton.RTM. and similar fluoroelastomers possess some chemical resistivity, they generally are still relatively susceptible to erosion by plasma-generated reactive species. Over time, the constant attack of these reactive species can break off portions of sealing o-ring 130. These portions may then be swept into reaction chamber 114 where they can cause damage serious enough to prevent the formation of functioning integrated circuits. Eventually, plasma-generated reactive species can even erode enough of sealing o-ring 130 to cause its complete failure. When used as sealing o-ring 130 in reactor system 100, such o-rings often fail in less than three days--an undesirably short time.
Increased success has been obtained using materials such as Kalrez.RTM. (a perfluoroelastomer commercially available from DuPont Dow elastomers) and Chemraz.RTM. (a perfluoroelastomer commercially available from Green, Tweed & Co, Kulpsville, Pa.). Because of the enhanced chemical resistivity of these materials, a sealing o-ring made of such perfluoroelastomer typically lasts longer than one made of a fluoroelastomer like Viton.RTM.. These materials are more expensive than Viton.RTM., however, and still often fail in less than a week of operation.
One type of o-ring that has been able to significantly increase the time before erosion-induced failure of sealing o-ring 130 is an o-ring encapsulated with Teflon.RTM. (a highly chemically resistant fluorocarbon polymer commercially available from E.I. du Pont de Nemours and Company). Teflon.RTM.-encapsulated o-rings typically include a Teflon.RTM. jacket that surrounds an elastomer core. These o-rings are substantially more resistant to erosion by plasma-generated reactive species than the elastomeric materials mentioned above, and thus may withstand the attack of plasma-generated reactive species for a significantly longer time.
Unfortunately, Teflon.RTM.-encapsulated o-rings are not well suited for use as sealing o-ring 130. For one, the Teflon.RTM. jacket of these o-rings makes these o-rings less resilient than many elastomeric o-rings. The inflexibility of Teflon.RTM.-encapsulated o-rings compared to o-rings composed of elastomeric materials can increase the difficulty of coupling plasma tube 104 and inlet conduit 112. And because of the relative lack of resiliency in the Teflon.RTM. jacket, a Teflon.RTM.-encapsulated o-ring may not be able to fully return to its original shape after being compressed during an operation cycle. Thus, a Teflon.RTM.-encapsulated sealing o-ring may become substantially deformed over numerous compression and expansion cycles. The discrepancy between the sealing o-ring's original shape and its deformed shape can significantly reduce the sealing ability of the o-ring. Eventually, a Teflon.RTM.-encapsulated sealing o-ring 130 may become so deformed that it can no longer provide the necessary sealing level. Even worse, the buildup of microstresses in the Teflon.RTM. jacket of the o-ring can cause the o-ring jacket to crack, potentially creating an immediate and unexpected loss of vacuum.
Furthermore, tube extension 124 of plasma tube 104 is undesirably susceptible to breakage, thus further reducing the mean time between failure reactor system 100. As noted above, tube extension 124 of plasma tube 104 is fixably attached to initial portion 120, and thus is incapable of significant independent movement. In addition, when the plasma tube is coupled to inlet conduit 112, tube extension 124 extends relatively far into throat 133 (to provide a clean path for the plasma). The position of tube extension 124 within throat 133 when the plasma tube and inlet conduit are coupled may be such that even slight movement of the inlet conduit relative to the plasma tube can cause coupling portion 126 to exert a large amount of stress on tube extension 124, particularly around glass weld 123. Unfortunately, such movement often occurs during coupling and decoupling of the plasma tube and inlet conduit at a level sufficient to cause the tube extension to break off from the rest of plasma tube 104. Such breakage typically occurs at or near welds 123.
Breakage of tube extension 124 may be highly disadvantageous for several reasons. For one, when tube extension 124 breaks plasma tube 104 typically is rendered unusable, so the entire plasma tube must be replaced. As might be expected, replacement of the entire plasma tube can be expensive, and can cost significantly more than the replacement of just one sealing o-ring. Further, reactor system 100 must be taken down for at least the time required to remove plasma tube 104, and in the case where a spare plasma tube is not immediately available, until a suitable replacement can be acquired or the broken plasma tube can be repaired. The processing time lost under these circumstances can significantly reduce reactor throughput--and thus overall process profitability.
Therefore, it would be desirable to develop a downstream plasma reactor system with an improved plasma tube sealing configuration. The desired reactor system should significantly extend the mean time between failure of a seal between the plasma tube and an inlet conduit to a reaction chamber. In addition, the desired system should have a plasma tube that is more durable, and in particular one that is less susceptible to breakage around its discharge opening. The improved sealing configuration should be one that can be incorporated without significantly increasing the difficulty of coupling the plasma tube to an inlet conduit or reducing the seal quality therebetween.